Method for repairing silver image glass photomasks with Ni

ABSTRACT

A method for repairing defects in the form of discontinuities in a photomask pattern in or on a glass substrate by means of electroless deposition of a nickel-containing coating on the surface of a glass substrate is disclosed.

This is a division of application Ser. No. 305,430, filed Sept. 25, 1981now U.S. Pat. No. 4,383,016.

FIELD OF THE INVENTION

The present invention is related generally to the art of producing glassphotomasks, and more particularly to the art of repairing defects in thephotomask pattern.

BACKGROUND OF THE INVENTION

Photomasks are used in photolithographic processes for printingmicroelectronic circuits and other precision photofabricated parts. In atypical photolithographic process, a substrate is covered with a layerof photoresist material in a desired pattern which is photographicallydeveloped by superimposing a photomask over the photoresist material.The photomask has a pattern of opaque and transparent areas with respectto actinic radiation which is passed through the photomask to developthe pattern in the photoresist material. Typically, the actinicradiation is ultraviolet light. The pattern is developed in thephotoresist material as a relief image by means of differingsolubilities of the exposed and unexposed portions of the photoresistmaterial. Since the preparation of a photomask involves the investmentof a substantial amount of time, labor and materials, it is desirablethat a photomask be sufficiently durable for repeated use in themanufacture of photofabricated articles. It is also desirable tomaximize the resolution of the pattern carried by a photomask in orderto improve the precision of the image it transfers to thephotofabricated articles.

Some photomasks consist of sheets of glass bearing patterned coatings ofphotographic emulsion, iron oxide or chromium. Iron oxide and chromiumare considerably more durable than photographic emulsions; however, allcoating patterned photomasks are subject to scratching and othermechanical damage which shortens their useful life. Photomasks ofimproved durability comprising a stained pattern within a glasssubstrate are disclosed in U.S. Pat. No. 3,573,948 to Tarnopol and U.S.Pat. No. 3,732,792 to Tarnopol et al. Although these stained glassphotomasks have improved durability, the step of etching a patternthrough a stained layer of the glass in the former, or the step ofetching through a tin oxide coating in the latter, results ininsufficient resolution for some applications.

U.S. Pat. No. 3,561,963 to Kiba discloses a stained glass photomaskwherein the desired pattern is etched into a copper film on a glasssubstrate and copper ions are subsequently migrated into the glass byheating. Although the stained photomask pattern is more durable than acoating, resolution is compromised in this process as a result of thethermal migration step which results in lateral spreading of the stainedareas into the adjacent unstained areas.

U.S. Pat. No. 2,927,042 to Hall et al and U.S. Pat. No. 3,620,795 toKiba disclose methods to minimize the lateral diffusion of staining inthe aforementioned processes. The Hall patent describes depositing afilm of stain-producing metal onto glass and removing portions of thefilm by photoetching. An electrical field is then passed through theglass so that the patterned film migrates into the glass substrate. TheKiba patent discloses etching a pattern into a metal film and migratingstain producing ions through apertures in the metal film by heating inan electrical field. Both methods suffer a loss of resolution as aresult of the etching step.

U.S. Pat. Nos. 2,732,298 and 2,911,749 to Stookey both disclose theproduction of a stained image within a glass plate by heating adeveloped silver-containing photographic emulsion on the glass. However,the use of relatively high temperature of 400° to 650° C. results in aloss of resolution of the stained pattern.

U.S. Pat. No. 4,155,735 to Ernsberger discloses an improved method formaking stained glass photomasks. The method comprises developing apatterned photoresist layer on a glass substrate and then applying anelectric field to enhance the migration of staining ions throughapertures in the photoresist pattern into the surface of the glasssubstrate.

In all of the above-described methods for producing patterns in or onphotomask substrates, defects can occur in the form of discontinuitiesin the photomask pattern as a result of dirt on the substrate, orimpurities in the photoresist or in the staining material. Since theproduction of a photomask requires considerable investment of time,labor and materials, it is desirable to repair such defects rather thanto reject the photomask plate. The present invention provides a methodfor repairing such defects in photomask patterns, whether these patternsare in the form of coatings on glass substrates or stained patternswithin a glass substrate.

SUMMARY OF THE INVENTION

The present invention particularly provides a method for eliminating pinhole and line defects within the line and pad structure of the photomaskpattern in a glass photomask assembly. The defect area is preferablyrepaired by depositing a metal coating over the defect. The metaldeposit may be formed by placing a drop of an electroless metal coatingsolution followed by a drop of reducing solution on the defect area.This metal deposition process may be repeated until a density sufficientto block actinic radiation is obtained. In an alternative process, theelectroless metal coating solution and reducing solution may be used todeposit a metal coating having a surface resistivity of about 100 ohmsper square. This area may then serve as a plating cathode for theelectroplating of metal to an effective radiation masking density. Afterthe metal coating is applied over the defect area, the shape of thedeposit may be precisely trimmed to the original pattern. Heat treatmentof the coating may be performed to increase its durability.

In accordance with the present invention, the defect area is thoroughlycleaned, if necessary, to remove any dirt or oily residue from thesubstrate surface. In a typical case wherein the substrate is glass, thesurface is made receptive to the electroless deposition of a metal filmby sensitizing with tin chloride solution followed by activation with apalladium chloride solution. The receptive glass surface is thencontacted with a drop of an electroless nickel plating solution and adrop of sodium borohydride reducing solution. After reaction, the areais rinsed and blown dry. This electroless deposition process ispreferably repeated until a sufficient density is obtained to mask theactinic radiation. A density of about 2.0 is desirable for maskingultraviolet radiation. The repaired photomask will function in the samemanner as a defect-free pattern. In an alternative embodiment, a drop ofan electroless nickel plating solution is followed by a drop of a sodiumborohydride reducing solution, and the reaction sequence repeated untila surface resistivity of about 100 ohms per square is obtained. Thecoated area is sufficiently conductive to serve as a plating cathode. Amobile electroplating cell may be placed over this cathode area andconnected to a DC power supply. The defect area is subjected toelectroplating until a sufficient masking density is obtained. Theelectroplating method requires less time than a strictly electrolessplating technique. After a metal coating of a sufficient masking densityis obtained by either method, the repaired area may be trimmed to matchthe original pattern. Trimming of excess metal deposit may be achievedby scraping with a metal razor knife or by dissolving the excess coatingby utilizing dilute hydrochloric acid. To avoid the need for trimming,the defect area to be repaired may be defined by placing strips of tapealong the pattern line. After deposition of the metal, a post-depositionheat treatment cycle may be employed to enhance the durability of thecoating. For example, the coating may be heated for about 10 to 60minutes at a temperature of about 250° to 350° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention may be used to repair defects in photomaskpatterns which comprise either coatings on a variety of substrates orstained patterns within a glass substrate, the photomasks employed inaccordance with the present invention are preferably prepared accordingto the technique disclosed in U.S. Pat. No. 4,144,066, U.S. Pat. No.4,155,735 and U.S. application Ser. No. 80,875 filed Oct. 1, 1979, thedisclosures of which are incorporated herein by reference.

A high resolution stained glass photomask is made in accordance withthese techniques, preferably by developing a silver-containingphotographic emulsion on a glass substrate and migrating silver from thedeveloped photographic emulsion into the glass. Silver ions migrate intothe glass, displacing mobile cations, particularly alkali metal ionswhich migrate deeper into the glass substrate. The migrated silver ionsare then reduced to the elemental state and agglomerated intosubmicroscopic crystals within the glass which produce a stainedpattern. Both the reduction and agglomeration of silver ions areaccomplished by means of heating in the presence of a reducing agent.The reducing agent may be tin ions which are normally present near thesurface of float glass, in which case the reducing and agglomeratingsteps may be carried out in ambient air, or the reducing agent may be areducing atmosphere such as forming gas maintained in a heating chamber.If a reducing gas atmosphere is employed, the stained pattern may beproduced at a lower temperature, which results in less fictive shrinkageof the bulk glass and the photomask image. The glass substrates, withinwhich the photomask patterns to be repaired in accordance with thepresent invention are prepared, may be of any typical glass compositionwhich contains a sufficient quantity of mobile cations for displacementby the silver ions which are electromigrated into the glass to producethe stained photomask pattern. Most glass compositions containsufficient alkali metal ions, such as sodium, potassium and lithium, foruse in accordance with the present invention.

Photographic emulsions useful in producing stained photomask patternswhich may be repaired in accordance with the present invention are thosecapable of being developed to produce a residual layer of emulsion andsilver or silver halide which has sufficient electrical conductivity topermit electromigration of silver ions from the emulsion into the glasssubstrate. The emulsion should also have high resolution capability inorder to produce a high resolution photomask pattern. Standardphotographic techniques are employed to expose and develop thephotographic emulsion. A glass substrate bearing a film of photographicemulsion is exposed to actinic radiation through a master pattern inorder to form a latent image. which is subsequently developed in thephotographic emulsion by exposure to appropriate developing solutions,whereby the silver halide in the exposed areas is converted to colloidalsilver. Either a positive image or a negative image may be developed onthe substrate depending upon the type of photographic emulsion employedand the developing process. Details of the developing techniques forboth types of images are described in detail in the cited references.

In preferred embodiments of the present invention, an electrical fieldis employed to migrate silver ions from the developed photographicemulsion into the subjacent glass surface. Preferred voltages for theelectromigration step are high enough to migrate the necessary quantityof silver ions within a reasonable period of time, but low enough toavoid arcing around the edges of the glass substrate between the anodeand cathode layers, typically about 50 to 1000 volts. At ambienttemperatures, the rate of ion migration may be impractically slow atthese voltages. Therefore, moderately elevated temperatures, preferablyabove 100° C., are preferred in order to obtain adequate migration ofsilver ions within a reasonable time. Preferred temperatures include arange of about 100° to 350° C., preferably about 200° to 250° C., whilepreferred voltages are from about 200 to 700 volts.

Once a sufficient quantity of ionic silver has been electromigrated intothe glass substrate to the desired depth, the stain pattern is developedby reduction of the silver ions to the elemental state, andagglomeration into submicroscopic crystals by application of heat in thepresence of a reducing agent. The reducing agent may be metallic ionssuch as the tin ions inherently present in the surface layer of floatglass or other ions such as copper ions purposefully present in theglass surface for use as a reducing agent. When metallic ions are usedas the reducing agent, the reduction and agglomeration of silver toproduce a stained pattern may be carried out in the ambient atmosphere.In the alternative, if insufficient such ions are present for use as areducing agent, a reducing atmosphere such as hydrogen-containing gas,e.g. forming gas, in the heating chamber may be used to carry out thereduction and agglomeration of the silver ions. If a reducing atmosphereis used, the reduction and agglomeration of silver ions may be carriedout in a shorter period of time. In either case, the period of timerequired for the reduction and agglomeration of the silver ions is afunction of the temperature. Temperatures above about 400° C. aregenerally required for the reduction and agglomeration steps to proceedat a reasonable rate in an ambient atmosphere, while temperatures belowabout 525° C. are necessary to avoid deformation of the glass substrate.Optimum temperatures are in the range of about 475° to 525° C., at whichtemperatures reduction and agglomeration of the silver ions to produce asatisfactory stained photomask pattern can be accomplished in a periodof about 15 minutes using metallic ions in the glass substrate as thereducing agent. When a reducing atmosphere is used as the reducingagent, practical rates of reduction and agglomeration of silver ions canbe obtained at lower temperatures in the range of 300° to 400° C. Theselower temperatures provide the advantage of minimizing lateral diffusionof the staining ions, and thereby maximizing resolution of the photomaskpattern. A combination of a reducing gas atmosphere and an internalionic reducing agent, such as tin, may be used to produce stainedphotomask patterns having enhanced optical and UV density.

After preparation of the photomask pattern is completed, the glassphotomask plate is inspected for defects. Such defects may appear asdiscontinuities in the photomask pattern, such as pin holes or linedefects. Relatively small defects, particularly in intricate patterns,may be sufficient to cause rejection of the photomask plate bycommercial standards. Since a substantial amount of time, work andmaterial has been expended in the production of a photomask plate, it isdesirable to repair such defects. In accordance with the presentinvention, such defects are repaired by depositing a metal coating,preferably a nickel-containing coating, over the defect area to asufficient thickness to mask the actinic radiation to which thephotomask plate will be exposed. An electroless deposition techniquesuch as the process described in U.S. Pat. No. 3,672,929 to Miller, thedisclosure of which is incorporated herein by reference, is preferablyemployed. While various metallic coatings might be deposited,nickel-containing coatings, especially those deposited withboron-containing reducing agents, are preferred for their durability.

The glass surface of the defect area must have good wetting properties.Therefore, if necessary, the defect area should be cleaned with adetergent solution. The cleaned surface should then be rinsed and dried.The glass surface is preferably sensitized by applying a drop of tinchloride solution, which is allowed to stand for one or two minutes,then a drop of sodium borohydride solution. The sensitized surface ispreferably rinsed and blown dry. The sensitized surface is thenpreferably activated by placing a drop of palladium chloride solution onthe defect area, and adding a drop of electroless nickel platingsolution and a drop of sodium borohydride reducing agent solution intothe droplet of palladium chloride solution. The reaction should beallowed to continue until bubbling ceases and a black precipitate isformed within the solution. The area is preferably rinsed and blown dry.A drop of electroless nickel plating solution and a drop of borohydridereducing solution are applied to the activated surface over the defectarea. The reaction is allowed to continue until bubbling ceases and ablack precipitate is formed within the solution. The area is preferablyrinsed and blown dry.

In accordance with one preferred embodiment of the present invention,the electroless plating steps are repeated until a deposit with adensity of at least 2.0 with respect to ultraviolet radiation isobtained. In another preferred embodiment of the invention, the stepsare repeated until a deposit is formed with a density of about 0.8 withrespect to ultraviolet radiation. This deposit, while insufficient torepair the defect, provides a nickel coating sufficiently conductive toserve as an electrode for the electrolytic deposition of anickel-containing film until a UV density of at least 2.0 is obtained.In either case, after a deposit with a UV density of at least 2.0 isobtained, the excess deposit may be trimmed away either with a metallicrazor knife or by means of a wooden stylus dipped in dilute hydrochloricacid. After the excess deposit is trimmed away, the repaired defect areapreferably is rinsed and blown dry. In another preferred embodiment ofthe present invention, the trimming techniques may be avoided by maskingthe defect area, e.g. by placing strips of tape along either side of thedefect, and then conducting the plating steps to deposit a coating withan effective masking density.

The tin chloride sensitizing solution is preferably a solution of aboutone gram of stannous chloride in 100 milliliters of deionized water. Thesodium borohydride solution is preferably a solution of 0.375 gram ofsodium borohydride in 500 milliliters of deionized water, to which maybe added sodium hydroxide to obtain a pH between about 11 and 11.3. Thepalladium chloride activating solution is used in concentrated form. Itis preferred to use the palladium chloride activating solution, theelectroless nickel plating solution and the sodium borohydride reducingsolution all at temperatures in the range of 110° to 115° F. (about 43°to 46° C.). If a post-deposition heat treatment is performed, thecoating is preferably heated for about 20 to 40 minutes at a temperatureof about 300° C.

The present invention will be further understood from the descriptionsof specific examples which follow.

EXAMPLE I

A stained glass photomask plate is inspected for defects. A defect areais repaired by sensitizing the glass surface above the defect with asolution of stannous chloride (1 gram/100 milliliters of water). Thesensitized area is activated with a solution of palladium chloride (0.25gram/100 milliliters of water). The surface, now receptive toelectroless deposition of a nickel-containing coating, is contacted witha drop of nickel plating solution (5 grams of nickel acetate/liter ofwater) and a drop of reducing solution (0.375 gram of sodiumborohydride/500 milliliters of water), both at a temperature of about115° F. (about 46° C.). After the solutions have reacted, the surface isrinsed and blown dry. The plating step is repeated four times to yield adeposit having a luminous transmittance of about 2.0 percent and anultraviolet light density of 2.3 measured with a Macbeth UVdensitometer.

EXAMPLE II

A defect is repaired following the procedure of Example I except thatthe reducing solution contains 0.5 gram per liter of sodium borohydride.After eight applications, a coating with a luminous transmittance of 2.0percent and a UV density of 2.21 is formed.

EXAMPLE III

A commercial glass photomask is inspected for defects. A defect area iscleaned with detergent solution to provide adequate wetting. A drop ofstannous chloride solution (1 gram/100 milliliters deionized water) isplaced over the defect area and allowed to stand for 1 to 2 minutes. Adrop of sodium borohydride solution (0.375 gram/500 milliliters ofwater) is added before the surface is rinsed and blown dry. Thesensitized area is contacted with a drop of palladium chloride solution(0.25 gram/liter deionized water) to which is added a drop ofelectroless nickel plating solution (prepared according to the NIKLAD740 electroless nickel process with reagents available fromALLIED-KELITE of Des Plaines, Ill.) and a drop of sodium borohydridesolution, both at 115° F. (about 46° C.). Reaction of the solutions isobservable by the formation of bubbles and the precipitation of a blackdeposit. When the reaction ceases, the area is rinsed and blown dry. Thearea is contacted with a drop of nickel plating solution and sodiumborohydride reducing solution as in the previous examples until adeposit having a UV density of 2.0 is formed.

EXAMPLE IV

A defect area in a glass photomask is treated as in Example III until adeposit with a UV density of about 0.8 is formed. The coated defect areais contacted with about 3 additional drops of nickel plating solution. Anegative electrode (metal probe-cathode) is placed onto the nickelcoating while a positive electrode (graphite rod-anode) isintermittently placed in contact with solution. The electroplatingprocess is continued until a coating with a UV density of at least 2.0is obtained. The repaired defect area is finally rinsed and blown dry.

The examples above are offered to illustrate the concept of the presentinvention. Various modifications such as the use of other well knownelectroless plating solutions, reducing agents and so on are includedwithin the scope of the invention which is defined by the followingclaims.

I claim:
 1. A patterned photomask article comprising a glass substrateand a silver stained pattern wherein defects in said pattern are coatedwith a nickel coating of sufficient density to mask actinic radiation.